Temperature probe and method for manufacturing a temperature probe

ABSTRACT

A temperature probe for determining the temperature according to the three-point probe method includes a three-wire line several meters long consisting of a first connecting line, a second connecting line, and a third connecting line connected to sensor element. The connecting lines are made of a first material and serve to transmit energy and the measured temperature values. A conductive element made of a second material is inserted in the second connecting line and in the third connecting line. The resistivity of said second material is higher than the resistivity of the first material. The two inserted conductive elements are designed in such that the second connecting line and the third connecting have the same resistance as the first connecting line. Additionally, the present disclosure refers to a method describing the manufacture of a temperature probe.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the priority benefit ofEuropean Patent Application No. 21215112.0, filed on Dec. 16, 2021, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is related to a temperature probe for highlyaccurate measurements.

BACKGROUND

Thermometers with resistance elements often include so-called thin-filmsensor elements, Resistance Temperature Detectors (RTD). Typically, sucha sensor element has a carrier substrate that is provided with leadwires and is metallically coated on a front surface. A metallic coatingmay also be available on the rear surface. Platinum elements are oftenused as sensor elements, which are also commercially available under thedesignations PT10, PT100, and PT1000, among others. In many cases, thesensor elements are encapsulated or embedded, preferably in pure ceramicpowders, and the connecting leads are guided in a guide tube toelectronics, for example a temperature transmitter. Details ofindustrial platinum resistance thermometers can be found, for example,in the European standard EN60751.

Various measuring methods for determining the temperature by means of aresistance element have become known from the prior art. A transmitsignal, usually in the form of an electric current, is impressed intothe sensor element and a receive signal, usually in the form of avoltage dropped across the sensor element, is detected, and evaluatedwith respect to temperature.

In the simplest case, the temperature is determined by means of atemperature-dependent resistance, in which the sensor element is simplycontacted via two connecting wires. The disadvantage of this solution isthat the resistance of the connecting wires is included in thetemperature determination as an error. According to another method, thesensor element is contacted via three connecting wires. By tapping thefalling voltage in pairs between two of the three connecting wires ineach case, a resistance of the connecting wires or the connecting linescan be largely compensated, if it can be assumed that the threeconnecting wires have the same resistance. It is also known to measurethe temperature with four connecting wires.

RTD Pt100 temperature sensors are widely used as sensor elements inprocess monitoring. They are the most common used standard temperaturesensors in the market. In certain application of process automation verylong sensors are required to reach the zone where the temperature shouldbe detected and/or monitored. Such temperature sensors can have a totallength up to a hundred of meters.

In such an application, the sensor cable must ensure the necessaryrobustness. Usually, it is made by an MgO cable with an external metalprotection sheath. The protection sheath is usually made of stainlesssteel or nickel alloy. A certain number of conductive wires, in mostapplications copper wires, form the connection wires for connecting thetemperature sensor to electronics.

The best-known solution of measuring the temperature in the case of verylong temperature probes consists in measuring the electric resistance ofthe Pt100 probe by a 4 wires terminal sensing. This so-called4-points-probe-method consists in injecting a current by using two wiresand measuring the voltage by using the remaining two wires as it isschematically described in FIG. 1 . The advantage of this measuringmethod is that the measurement is not affected by the resistance of theconnection wires between the probe and electronics of the temperatureprobe and therefore the measurement is independent of the length of theconnection cable.

Another very common measuring method requested by the market is the 3wires terminal sensing, 3-wires-connection, or 3 points probe method.Generally, this method is requested to reduce the costs of thetemperature measuring device, or for design reasons: indeed, in probeswith multiple sensing elements (two or more), the reduction of thenumber of necessary wires in the cable has the benefit to make the cablemore compact (smaller diameter and so less invasive) or allows toincrease the number of possible measuring points in the same cable.

Compared to the 4 wires, this method has an essential limit: themeasurement can compensate the resistance of the connection wireswithout any additional error only if the resistance of the cable is thesame in all three cables used in the measuring device.

This limit can easily be demonstrated by analyzing how the measurementis performed—see FIG. 2 :

In this case the resistance of the Pt100 probe is the result of theresistances of two loops:

In one circuit the resistance Rc1 is measured between the commonconnection point C and the point 1:

Rc1=R common+R1+R Pt100.

Then the measurement is made between points 1 and 2:

Rc2=R1+R2

Under the assumption that the three connection resistances are the samethe result is calculated as:

Measurement=Rc1−Rc2=R common+R1+R Pt100−(R1+R2)=R Pt100+R common−R2

Other calculations methods are possible and applied by differentmeasuring devices, but the result is always the same: the measuringdevice can measure the Pt100 without any error only if the three cableshave the same resistance. This becomes clear when considering thecircuit shown in FIG. 3 .

Measure 1=Va/I=R common+R Pt100 (in R2 the current is zero)

Measure 2=Vb/I=R1

Measure final=(Va−Vb)/I=R common+R Pt100−R1=Pt100 (if R common=R1)

In general, this method is a good compromise, but it assumes that theresistances of the connection cables are the same. This is usually nottrue. Unfortunately, the wires of an MgO cable do not have the sameresistance. This is caused by the manufacturing drawing process of thewires. In these components it is common to have a resistance differenceof about 1-3% of the total value that is depending on the length of thecable. For very long sensors, the total resistance could reach 20 Ohmsor more depending also on the diameter of the cable, i.e., of thediameter of the internal wires.

The difference of resistance among the wires of the cable can negativelyaffect the measurement accuracy that can be over the requested limits.

The measurement error of a Pt100 probe can be as follows if acalibration check in ice+water reference bath at 0° C. is made:

Temperature error=(R Pt100−100)/0.39=Resistance error/0.39 0°

Class AA: max allowable T error<0.10° C.; max allowable R error<0.039Ω

Class A: max allowable T error<0.15° C.; max allowable R error<0.0585Ω

Class B: max allowable T error<0.10° C.; max allowable R error<0.117Ω

These measuring errors of the different classes are generally notachievable with very long MgO cable constructions if using the known 3wires methods.

SUMMARY

It is an object of the present disclosure to provide a temperature probeoperating according to a three-wire method, which enables highlyaccurate temperature measurement. Additionally, it is an object of thepresent disclosure to provide a method to proc

To achieve this object, the present disclosure comprises a temperatureprobe for determining the temperature according to the three-point probemethod with a sensor element providing temperature values, wherein athree-wire line of several meters, consisting of a first connectingline, a second connecting line and a third connecting line, isassociated with the sensor element, wherein the connecting lines aremade of a first material and serve to transmit energy and measuredtemperature values, wherein a conductive element made of a secondmaterial is inserted in each of the second connecting line and the thirdconnecting line, the resistivity of said second material is greater thanthe resistivity of the first material, and wherein the insertedconductive elements are designed in such a way that the secondconnecting line and the third connecting line have substantially thesame resistance as the first connecting line.

The solution according to the present disclosure is particularlyapplicable in connection with 3-wire cables of quite long temperatureprobes to ensure a preferably high accuracy class: The resistancecompensation of the connecting lines is preferably reached by insertingquite short pieces of a conductive material having a higher resistivitythan the material of the connecting lines in usually two of the threewires of a 3-wire cable. By inserting conductive elements of a certainlength and/or diameter into two of the three wires, it is achieved thatthe resistance of each of the three connecting wires is equal.

According to an embodiment of the temperature probe it is proposed thatthe resistivity of the second material is at least five times higherthan the resistivity of the first material. Preferably, the connectinglines are made of copper, and the inserted conductive elements are madeof constantan. The resistance compensation is achieved by selecting theright material for the conductive element. To achieve an effectiveconstruction, a good compensation and a short length, the material musthave an electrical resistivity much higher than the original wires. Mostof the wires of the MgO cables are made of copper. An analysis done bycomparing different materials and the resistance values that must becompensated, leads to consider Constantan as the preferred material ofthe conductive elements. Constantan has a high resistivity compared toe.g. copper and good and robust mechanical properties.

According to an embodiment of the temperature probe the insertedconductive elements made of the at least one second material arearranged within a transition bushing of the probe. In the transitionbushing, two sections of the three-wire cable are connected together. Toprovide extended temperature probes, a transition bushing is generallyused to connect the MgO cable to a flexible extension cable. Theresistance compensating conductive elements are inserted between the endsections of the corresponding wires of the MgO cable and the flexibeleextension cable. They can be connected by any of the known methods, forexample: welding, brazing, soldering, or crimping. Every connection canbe protected by an additional Kapton or thermo-shrinking insulationcable to isolate it from the other connections. Finally, the completebushing may be sealed by a resin potting.

According to an alternative design of the temperature sensor, theconductive elements made of at least a second material are inserted in aconnection area through which the three-wire line can be connected toexternal electronics: the conductive elements that compensate for thedifferences in resistance of the connection lines are attached to theterminals to which the connection lines of the main cable are connected.Depending on the length of the main cable, this can be the MgO cable orthe flexible extension cable. The two wires into which the conductiveelements are inserted may be stripped and interrupted. The conductiveelements are inserted between the connecting wires and the terminals.Again, the connections can be welded, brazed, soldered, or crimped. Thewires are insulated from each other, e.g., with heat shrink tubing.Additional shrink tubing insulation can be applied to protect theconnection.

According to another alternative design of the temperature sensor, theconductive elements made of at least a second material are inserted in aconnection area. Preferably this connection area is arranged within theflexible extension cable. The two wires into which the conductiveelements are inserted may be stripped and interrupted. The conductiveelements are inserted between the connecting wires and the terminals.Again, the connections can be welded, brazed, soldered, or crimped. Theconductive elements and the wire connections are attached to theterminals directly like in the previous embodiment or by using a rigidsupport as a reinforcement.

According to an embodiment of the temperature probe, the resistance ofeach of the two conductive elements inserted in the second connectingline and in the third connecting line is designed in such a way that thetemperature probe provides measured values with a predeterminedmeasurement accuracy. For example, the accuracy class may be A or B.

It is further provided that the sensing element is a ResistanceTemperature Detector—RTD—element, preferably a platinum measuringresistor PT100. Any other appropriate sensor element may be used inconnection with the inventive solution.

With regard to the method of producing a temperature probe fordetermining the temperature according to the three-point probe methodwith a sensor element, preferably designed as a platinum measuringresistor, which provides temperature measured values, wherein athree-wire line several meters long, consisting of a first connectingline, a second connecting line and a third connecting line, isassociated with the sensor element, wherein the connecting lines aremade of a first material with a predetermined specific resistance andserve for transmitting energy and for transmitting the measuredtemperature values, the following steps are proposed:

-   -   measuring the resistance of each of the three connecting lines;    -   determining the connecting line with the highest resistance,        hereinafter: the first connecting line;    -   inserting a first conductive element into the second connecting        line, wherein the first conductive element is made of a second        material having a resistivity greater than the resistivity of        the first material, and wherein the inserted first conductive        element is dimensioned such that the second connecting line has        the same resistance as the first connecting line;    -   inserting a second conductive element into the third connecting        line, wherein the second conductive element is made of the        second material, and wherein the inserted second conductive        element is dimensioned such that the third connecting line has        the same resistance as the first connecting line.

In a development of the method the conductive elements are welded,brazed, soldered, or crimped for insertion into the correspondingconnecting lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is explained in more detail with reference to thefollowing figures.

FIG. 1 shows a circuit for measuring temperature according to the4-points-probe method, and an schematic view of a correspondingtemperature probe,

FIG. 2 shows a circuit for measuring temperature according to the3-points-probe method, and a schematic view of a correspondingtemperature probe,

FIG. 3 shows in ore details the circuit of FIG. 2 ,

FIG. 4 shows a table of the measurement error as function of the lengthof an exemplary cable with three connecting wires,

FIG. 5 shows schematically an inventive temperature probe,

FIG. 6 shows a table of the resistivity of different conductivematerials,

FIG. 7 shows a first embodiment of the inventive temperature probe,

FIG. 8 shows a second embodiment of the inventive temperature probe, and

FIG. 9 shows a third embodiment of the inventive temperature probe.

DETAILED DESCRIPTION

The different prior art solutions of temperature probes 1 and thecorresponding methods for measuring the temperature are alreadydescribed in FIGS. 1-3 .

For temperature sensors 1 with resistance thermometer elements 2, forexample a Pt100, MgO cables 14 are usually used. A cable length of morethan 50 m is often required to measure the temperature in a remotelocation. Further requirements are a predetermined high measuringaccuracy (e.g., class A) and the use of a 3-wire line. Due to thetechnical properties of the MgO cable, it is difficult, or in some casesimpossible, to reach the requested accuracy class. The problem is thatthe inner connecting wires 4, 5, 6 of an MgO cable 14 usually do nothave the same resistance. The manufacturers generally declare anaccuracy among the wires 4, 5, 6 of a three-wire cable 3 of about 0.002Ohm/m on a typical 6 mm MgO cable 14 with a wire resistance of about0.04-0.06 Ohm/m.

Corresponding experimental investigations have confirmed thatstatistically a difference in resistance of wires 4, 5, 6 with astandard deviation of about 1% of the total measured value can beexpected.

FIG. 4 shows, as an example, a table visualizing the measurement erroras a function of the length of a three-wire cable 3 of a temperatureprobe 1 with three connecting wires 4, 5, 6. In particular, the tableshows the maximum cable length above which a required accuracy class Aor B for the temperature measurement can no longer be maintained. So, byconsidering very long temperature probes 1 we have a situation like itis shown in FIG. 4 : The measurement values leave a given accuracy classas soon as the connecting lines 4, 5, 6 exceed a certain length.

According to the inventive temperature probe 1 the differences of theresistances of the three connecting lines 4, 5, 6 is compensated byadding an additional resistance. Preferably, the resistances of two ofthe three wires 4, 5, 6 are equalized to the resistance of theconnecting line (for example 4) with the highest resistance. Theinventive temperature probe 1 is simple and inexpensive to manufacture,as the compensation method is less invasive, but provides a highaccuracy of the temperature measurement. A piece of a conductive element7, 8 with a higher resistivity and the determined dimensions is neededto modify the resistance of the remaining two connecting lines 5, 6 insuch a way that each of the connecting lines 4, 5, 6 has the sameresistance.

FIG. 5 shows a schematic view of the inventive temperature probe 1determining the temperature according to the three-point probe method.FIG. 6 shows a table of the resistivity of different conductivematerials.

The calculation of the linear resistance of a connecting line 4, 5, 6 isquite simple:

Linear wire resistance=material resistivity/wire section.

By doing the calculation using a standard Constantan wire with adiameter between 0.2 and 0.5 mm we can compensate the resistancedifferences between the three connecting lines 4, 5, 6 of a MgO cable 14by adding a conductive element 7, 8 of 10 mm to 50 mm of a Constantanwire.

Whereby the length of the conductive element 7, 8 is calculated by:

Compensation length=Resistance difference/Linear wire resistance.

In the following the steps for compensating resistance differences onthe three wires is described:

The process starts by measuring the resistance of each of the threewires 4, 5, 6.

The wire 4 with the highest resistance is identified and the differencebetween the maximum value and the resistance values of the two remainingwires 5, 6 is calculated.

-   -   The compensation length of the conductive element 7, 8,        preferably made of Constantan, is calculated for each of the two        remaining wires 5, 6 to equalize the resistance of each of them        with the resistance of the first wire 4 with the highest        resistance value.

FIG. 7 shows a first embodiment of the temperature probe 1 according tothe present disclosure. The focus is on the attachment of the conductiveelements 7, 8 to two or at least one of the connecting wires 4, 5, 6.The conductive elements 7, 8 made of the at least one second material,for example constantan, are arranged in a transition bushing 9 of thetemperature probe 1. Such a transition bushing 9 serves to connect twodifferent sections 10, 11 of the three-wire cable 3.

For extended temperature probes 1 such a transition bushing 9 isgenerally used to connect the MgO 14 cable to a flexible extension cable15. The compensating conductive elements 7, 8 are inserted between theend sections of the corresponding wires of the MgO cable 14 and theflexible extension cable 15. They can be connected by any of the knownmethods, for example: welding, brazing, soldering, or crimping. Forelectrical insulation, each joint may be protected by an additionalKapton or heat shrink insulating sleeve or cover 16. Finally, thecomplete bushing 9 may be sealed by a resin potting 17.

FIG. 8 shows a view on a second embodiment of the inventive temperatureprobe. According to this alternative design of the temperature probe,the conductive elements 7, 8 made of at least a second material areinserted in a connection area 12 through which the three-wire cable 3can be connected to external electronics 13: the conductive elements 7,8 that compensate for the differences in resistance of the connectionlines 4, 5, 6 are attached to the terminals 18 to which the connectionlines 4, 5, 6 of the main cable 3 are connected. Depending on the lengthof the main three-wire cable 3, this may be the MgO cable 14 or theflexible extension cable 15. The two wires 4, 5, 6 into which theconductive elements 7, 8 of a determined design are inserted may bestripped and interrupted. The conductive elements 7, 8 are insertedbetween the connecting wires 4, 5, 6 and the terminals 18. Again, theconnections can be welded, brazed, soldered, or crimped. For electricalinsulation, each joint may be protected by an additional Kapton or heatshrink insulating sleeve or cover 16. Additionally, heat shrink tubinginsulation 19 can be applied to protect the connections.

FIG. 9 shows a third embodiment of the inventive temperature probe 1.Here the conductive elements , 8 made of the second material andcorresponding wire connections 20 connecting the conductive elements 7,8 to the wires 5, 6 are protected by an encapsulation box 21. Theconductive elements 7, 8 and the wire connections 20 are protected by anencapsulation box 21. Preferably tis encapsulation box is inserted inthe flexible extension cable 15. They can also be directly inserted intothe terminal part 19 by which the temperature probe 1 is connected to anexternal electronics (13). The conductive elements 7, 8 and the wireconnections 20 are inserted into proper heat shrink tubing 16 or aninsulating tape . Then the connection area 12 is protected by anadditional encapsulation box 21. The connection area 12 may be at theend or in the middle of the flexible extension cable 15.

1-12. (canceled)
 13. A temperature probe for determining a temperatureaccording to a three-point probe method, the temperature probecomprising: a sensor element embodied to provide temperature values; athree-wire line several meters long, the three-wire line including: afirst connecting line; a second connecting line; and a third connectingline, wherein the three-wire line is connected to the sensor element,and wherein each of the three connecting lines is made of a firstmaterial and serves to transmit energy and measured temperature values;a first conductive element made of a second material and inserted in thesecond connecting line; and a second conductive element made of thesecond material and inserted in the third connecting line, wherein aresistivity of the second material is higher than a resistivity of thefirst material, and wherein the two inserted conductive elements aredesigned such that the second connecting line and the third connectingline have a same resistance as the first connecting line.
 14. Thetemperature probe according to claim 13, wherein the resistivity of thesecond material is at least 5 times higher than the resistivity of thefirst material.
 15. The temperature probe according to claim 14, whereinthe three connecting lines are made of copper, and wherein the twoinserted conductive elements are made of constantan.
 16. The temperatureprobe according to claim 13, wherein the two inserted conductiveelements made of the second material are arranged within a bushing wheretwo sections of the three-wire line are interconnected.
 17. Thetemperature probe according to claim 13, wherein the two conductiveelements made of the second material are arranged in a connection areavia which the three-wire line is connectable to external electronics.18. The temperature probe according to claim 13, wherein the resistanceof the two conductive elements inserted in the second connecting lineand in the third connecting line is designed such that the temperatureprobe provides measured values with a predetermined measurementaccuracy.
 19. The temperature probe according to claim 13, wherein thesensing element is a Resistance Temperature Detector (RTD) element. 20.A method of manufacturing a temperature probe for determining atemperature according to a three-point probe method with a sensorelement designed as a platinum measuring resistor that providestemperature measured values, wherein a three-wire line several meterslong having a first connecting line, a second connecting line, and athird connecting line is associated with the sensor element, wherein thethree connecting lines are made of a first material with a predeterminedspecific resistance and serve for transmitting energy and fortransmitting the measured temperature values, the method comprising:measuring a resistance of each of the three connecting lines;determining a connecting line having the highest resistance,hereinafter: the first connecting line; inserting a first conductiveelement into the second connecting line, wherein the first conductiveelement is made of a second material having a resistivity greater thanthe resistivity of the first material, and wherein the inserted firstconductive element is dimensioned such that the second connecting linehas the same resistance as the first connecting line; inserting a secondconductive element into the third connecting line, wherein the secondconductive element is made of the second material, and wherein theinserted second conductive element is dimensioned such that the thirdconnecting line has the same resistance as the first connecting line.21. The method according to claim 20, wherein the two conductiveelements are welded, brazed, soldered, or crimped for insertion into thecorresponding connecting lines.
 22. The method according to claim 20,wherein the inserted conductive elements made of the second material arearranged within a bushing where two sections of the three-wire line areinterconnected.
 23. The method according to claim 20, wherein theinserted conductive elements made of the second material are arranged ina connection area via which the three-wire line is connectable toexternal electronics.
 24. The method according to claim 20, wherein theconductive elements made of the second material and corresponding wireconnections connecting the conductive elements to the wires areprotected by an encapsulation box, whereby the conductive elementsprotected by the encapsulation box are inserted in a flexible extensioncable.